Additional data confirm clinically important, statistically
significant, and lasting improvement in pain, disability, and quality of
life for people with Chronic Low Back Pain and limited treatment options

Mainstay Medical International plc (Mainstay or the Company, listed
on Euronext Paris: MSTY.PA and ESM of the Irish Stock Exchange:
MSTY.IE), a medical device company focused on bringing to market ReActiv8®,
a new implantable neurostimulation system to treat disabling Chronic Low
Back Pain (CLBP), today announced additional results from the ReActiv8-A
Clinical Trial, an international, multi-centre, prospective, single arm
trial to gather data for a submission for CE Marking for ReActiv8. The
additional results are consistent with those released on 31 August, 2015
and continue to show clinically important, statistically significant and
lasting improvement in pain, disability and quality of life in this
clinically challenging population.

NOTE: Results in this release have been updated including data
from 13 subjects who had not yet reached the 180 day follow-up at the
time of the August press release. A full description of the ReActiv8-A
Trial and updated results are provided in an addendum to this release.

ReActiv8 is for treatment of people who suffer from CLBP, have attempted
most or all available treatment options, and are not candidates for back
surgery or spinal cord stimulation. The ReActiv8-A Trial population was
relatively young (mean age 43.9 years) and had a long history of low
back pain (mean 13.8 years). All of the subjects had attempted physical
therapy, and 70% were taking opioids for back pain. The results
presented are based on data from the first 47 subjects implanted in the
ReActiv8-A Trial of whom 46 have reached the 90 day follow up and 45
have reached the 180 day follow up.

Results highlights:

Clinical performance of ReActiv8 at 90 days compared to baseline for
all subjects is:

63% with clinically important improvement in back pain
defined as ≥2 point reduction on the 0-10 Numerical Rating Scale
(NRS) for low back pain1 measured on the day.

57% responder rate for pain: A responder is defined as a
subject with a clinically important improvement in mean of prior 7
days NRS with no clinically significant increase in medications
taken for low back pain.

57% with a clinically important improvement2 in
disability on the Oswestry Disability Index (ODI).

67% with a clinically important improvement3 in
quality of life on the EQ-5D scale.

Clinical performance at 90 days is even better for the group of
subjects who do not receive financial compensation for being out of
work due to their back pain. For those 32 subjects the results are:

72% with clinically important improvement in low back pain
NRS on the day.

69% responder rate for pain.

63% with clinically important improvement in ODI.

69% with a clinically important improvement in EQ-5D.

Improvements in low back pain, disability and quality of life were
generally consistent or improved at 180 days (n=45). Paired data for
all subjects at 90 and 180 days respectively are:

63% and 58% with clinically important improvement in
low back pain NRS on the day.

57% and 60% with clinically important improvement in
ODI.

67% and 73% with clinically important improvement in
EQ-5D.

61% and 67% reported >50% Percent Pain Relief.

Adverse Events (AEs) incidence and type were comparable to AEs in
clinical trials reported for other neurostimulation devices, with no
unanticipated AEs and no serious AEs related to the device, therapy or
procedure.

The observed lead migration incidence (<1%) demonstrates that the
ReActiv8 lead mitigates the risk of lead migration identified with
commercially available neurostimulation leads in the earlier
Feasibility Trial.

In the August press release, the Company announced a modification of
the implant technique with different lead routing developed to
mitigate the risk of breaks of the wires inside the lead, which had
been observed in the ReActiv8-A Trial. Experience to date with the
first 14 subjects implanted with the modified approach (new implants
and revisions) are encouraging.

The Company announced on 2 November, 2015 that a submission for CE
Marking was made.

Subjects continue to be enrolled in the ReActiv8-A Trial to gather
additional data on performance and safety which the Company plans to
incorporate into the Post Market Clinical Follow Up. To date there have
been 6 additional subjects implanted in the ReActiv8-A Trial.

- ReActiv8-A Trial Results Below -

Addendum: ReActiv8-A Trial Description and Results

Subjects were enrolled in the ReActiv8-A Trial if they continued to
experience disabling CLBP despite at least 90 days of medical
management, which included at least physical therapy and drugs (most
subjects had attempted many other treatments including chiropractic,
massage, and acupuncture). In addition, subjects had no identifiable
spine pathology that could be the clear cause of their CLBP, had no
prior spine surgery, and were not candidates for spine surgery or spinal
cord stimulation. Back pain was assessed using a Numerical Rating Scale
(NRS) in which subjects were asked to grade their low back pain from 0
(no pain) to 10 (worst imaginable pain). Subjects were instructed to not
change their medications for low back pain until after the 90 day end
point.

Subjects were implanted with ReActiv8 in a short surgical procedure.
After approximately 14 days, subjects began stimulation sessions with
ReActiv8 for 30 minutes each morning and evening, and outcome data
collection is at 90 days, 180 days and annually thereafter.

Baseline Characteristics

The key baseline characteristics for all 47 implanted subjects are shown
in the table below.

Characteristic

Mean ± SD or n (%)

Age

43.9 ± 10.7

Gender (Male - Female)

21 (45%) - 26 (55%)

Duration of Back Pain (years)

13.8 ± 10.3

Average Back Pain NRS

6.9 ± 0.8

Disability on Oswestry Disability Index (ODI)

44.5 ± 10.5

Quality of Life on EQ-5D

0.47 ± 0.2

Back Pain Medications

Opioids

33 (70%)

Analgesics

28 (60%)

Non-Steroid Anti-Inflammatory Drugs (NSAIDS)

18 (38%)

In summary, this is a population of relatively young men and women who
have suffered from disabling low back pain for over a decade, have tried
many other treatment options, and most now depend on strong medications
for pain.

Outcomes Data

There were 47 subjects implanted, 46 reached the 90 day endpoint (one
subject was explanted) and 45 subjects reached the 180 day follow-up
point (one subject missed the 180 day follow-up). Data continue to be
collected, and are presented for all subjects who have reached the 90
and 180 day follow up to date. The table below highlights summary
outcome data. Note that for responder rates, the proportions are
presented as X/Y where X is the number of responders and Y is the number
of subjects who have been assessed at that endpoint.

Outcomes – N=47 – All Subjects

Day 90 (n=46)

Day 180 (n=45)

Low back pain – prior 7 day average Numerical Rating Scale (0 –
10)

Improvement from baseline – absolute

2.5 ± 0.3 p<0.0001

Not recorded beyond90 days per protocol

Improvement from baseline – %

36%

Responder Rate (% of subjects)

57% (26/46)

Low back pain – on the day Numerical Rating Scale (0 – 10)

Improvement from baseline - absolute

2.3 ± 0.3 p<0.0001

2.1 ± 0.4 p<0.0001

Improvement from baseline – %

33%

30%

% of subjects with ≥2 point improvement

63% (29/46)

58% (26/45)

Low back pain improvement – Percent Pain Relief (0 – 100%)

Reported change (%)

47% ± 4.3

52% ± 4.5

% of subjects with ≥50% pain relief

61% (28/46)

67% (30/45)

Disability (Oswestry Disability Index) (0 – 100)

Improvement from baseline

14.8 ± 2.3 p<0.0001

12.4 ± 2.4 p<0.0001

% of subjects with clinically important change

57% (26/46)

60% (27/45)

Quality of life (EQ-5D) (1 = Maximum Value)

Improvement from baseline

0.16 ± 0.03 p<0.0001

0.13 ± 0.03 p=0.0003

% of subjects with clinically important change

67% (31/46)

73% (33/45)

Outcomes – Pain

For the 90 day endpoint, the assessment was the mean of the prior seven
days of daily average low back pain NRS recorded in a Journal. For all
assessments (including after 90 days), subjects were asked to report
their low back pain NRS on the day. A “responder” is defined in
accordance with the IMMPACT recommendations4 as a subject
with a reduction in NRS of 2 points or more from baseline, with the
addition of no clinically significant increase in medications taken for
low back pain.

The responder rate at 90 days was 57%, and at 180 days 58% of subjects
reported ≥2 point improvement on the single day NRS.

Subjects were also asked to rate their “Percent Pain Relief” compared to
baseline. By this measure, 61% of subjects reported 50% or better
improvement at 90 days, and 67% reported 50% or better improvement by
180 days.

Outcome – Disability (ODI)

The Oswestry Disability Index (ODI) is a disease specific assessment of
the disabling effects of back pain. The IMMPACT recommendations are that
a reduction from baseline of 10 points or more constitutes an “important
change”.5

57% (26/46) of subjects had an important change in ODI at 90 days, and
the mean improvement was 14.8 points. At 180 days, 60% (27/45) had an
important change in ODI, and the mean improvement was 12.4 points.

Outcome – Quality of Life (EQ-5D)

The European Quality of Life Assessment (EQ-5D) is commonly used as an
outcome measure in studies on back pain.

67% (31/46) of subjects had a clinically important improvement in EQ-5D
at 90 days, and the mean improvement was 0.16 ± 0.03. At 180 days, 73%
(33/45) had a clinically important improvement in EQ-5D, with a mean
improvement of 0.13 ± 0.03.

Other Outcomes

The Treatment Satisfaction Questionnaire is an assessment of the
subject’s satisfaction with the treatment. At 90 days (n=46) 89% of
subjects were satisfied, of whom 85% were very satisfied and at 180 days
(n=45) 84% were satisfied of whom 87% were very satisfied.

The Clinical Global Impression is the physician’s assessment of the
subject compared to baseline, and at 90 days (n=46), 85% were rated as
“better.”

Effect of Financial Compensation

Analysis of the data showed significantly better outcomes in subjects
who were not receiving financial compensation for being out of work due
to their back pain. There is consistent evidence in scientific
literature that financial compensation for a change in work status due
to back pain is a strong predictor for treatment failure in clinical
trials of pain therapies. Therefore it has become usual practice in
clinical trials of therapies for pain (including recent spinal cord
stimulation trials) to exclude subjects with financial compensation.6,7

In the ReActiv8-A Trial, there were 15 subjects who were receiving
financial compensation for being out of work due to their back pain. The
90 day mean improvement in NRS for this group (the “Financial
Compensation Cohort)” was 1.4 points (21% improvement), and the
responder rate was 29% (4/14). In contrast, the outcomes for those not
receiving financial compensation (the “Usual Cohort”) were superior
(p=0.05 for the 90 day NRS outcome). The 90 day mean improvement in NRS
for the Usual Cohort was 3.0 points (43% improvement, and the responder
rate was 69% (22/32). Summary results for the Usual Cohort are presented
in the table below.

Outcomes – N=32 – Usual Cohort

Day 90 (n=32)

Day 180 (n=31)

Low back pain – prior 7 day average Numerical Rating Scale (0 –
10)

Improvement from baseline – absolute

3.0 ± 0.4 p<0.0001

Not recorded beyond90 days per protocol

Improvement from baseline – %

43%

Responder Rate (% of subjects)

69% (22/32)

Low back pain – on the day Numerical Rating Scale (0 – 10)

Improvement from baseline - absolute

2.8 ± 0.4 p<0.0001

2.4 ± 0.5 p<0.0001

Improvement from baseline – %

40%

35%

% of subjects with ≥2 point improvement

72% (23/32)

58% (18/31)

Low back pain improvement – Percent Pain Relief (0 – 100%)

Reported change (%)

50% ± 5.4

55% ± 5.6

% of subjects with ≥50% pain relief

63% (20/32)

71% (22/31)

Disability (Oswestry Disability Index) (0 – 100)

Improvement from baseline

16.5 ± 2.6 p<0.0001

12.0 ± 2.9 p=0.0002

% of subjects with clinically important change

63% (20/32)

58% (18/31)

Quality of life (EQ-5D) (1 = Maximum Value)

Improvement from baseline

0.19 ± 0.03 p<0.0001

0.14 ± 0.05 p=0.0054

% of subjects with clinically important change

69% (22/32)

77% (24/31)

Adverse Events

Adverse events are adjudicated by an independent Clinical Events
Committee. As of the most recent adjudication in November, there were 67
reported AEs related to the device, therapy or procedure. There were
three Serious Adverse Events (SAEs), none of which were related to the
device, therapy or procedure. There were no unanticipated AEs or
unanticipated device related AEs. The AE incidence and type were
comparable to AEs in clinical trials reported for other neurostimulation
devices.

Lead Related Events

To date, the ReActiv8 lead has shown only one instance of lead migration
(dislodgement) out of 134 implanted leads (including those implanted in
revision procedures) for a lead migration incidence of less than 1%.
These results demonstrate that the ReActiv8 lead mitigates the risk of
lead migration identified with commercially available neurostimulation
leads in the earlier Feasibility Trial.

In the August press release, the Company announced a modification of the
implant technique with different lead routing developed to mitigate the
risk of breaks of the wires inside the lead, which had been observed in
the ReActiv8-A Trial. Experience to date with the first 14 subjects
implanted with the modified approach (new implants and revisions) are
encouraging.

Summary

These results demonstrate that for a population of people with long term
Chronic Low Back Pain and limited treatment options, treatment with
ReActiv8 delivers clinically important, statistically significant, and
lasting improvement in pain, disability, and quality of life.

- End -

About Mainstay

Mainstay is a medical device company focused on bringing to market an
innovative implantable neurostimulation system, ReActiv8®,
for people with disabling Chronic Low Back Pain (CLBP). The Company is
headquartered in Dublin, Ireland. It has subsidiaries operating in
Ireland, the United States and Australia, and is listed on Euronext
Paris (MSTY.PA) and the ESM of the Irish Stock Exchange (MSTY.IE).

About the ReActiv8-A Trial

The ReActiv8-A clinical trial is a prospective single arm clinical trial
with up to 96 subjects at sites in Australia and Europe. Outcome
measures for the ReActiv8-A clinical trial are assessed at a three month
endpoint after activation of stimulation and compared to baseline prior
to implant. Further details can be obtained at https://clinicaltrials.gov/show/NCT01985230.

About Chronic Low Back Pain

One of the recognised root causes of CLBP is impaired control by the
nervous system of the muscles that dynamically stabilise the spine in
the lower back, and an unstable spine can lead to back pain. ReActiv8 is
designed to electrically stimulate the nerves responsible for
contracting these muscles and thereby help to restore muscle control and
improve dynamic spine stability, allowing the body to recover from CLBP.

People with CLBP usually have a greatly reduced quality of life and
score significantly higher on scales for pain, disability, depression,
anxiety and sleep disorders. Their pain and disability can persist
despite the best available medical treatments, and only a small
percentage of cases result from an identified pathological condition or
anatomical defect that may be correctable with spine surgery. Their
ability to work or be productive is seriously affected by the condition
and the resulting days lost from work, disability benefits and health
resource utilisation put a significant burden on individuals, families,
communities, industry, and governments.

ReActiv8 is an investigational device and is not approved for
commercialisation anywhere in the world.

CAUTION – in the United States, ReActiv8 is limited by federal law to
investigational use only.

Forward looking statements

This announcement includes statements that are, or may be deemed to be,
forward looking statements. These forward looking statements can be
identified by the use of forward looking terminology, including the
terms “anticipates”, “believes”, “estimates”, “expects”, “intends”,
“may”, “plans”, “projects”, “should” or “will”, or, in each case, their
negative or other variations or comparable terminology, or by
discussions of strategy, plans, objectives, goals, future events or
intentions. These forward looking statements include all matters that
are not historical facts. They appear throughout this announcement and
include, but are not limited to, statements regarding the Company’s
intentions, beliefs or current expectations concerning, among other
things, the Company’s results of operations, financial position,
prospects, financing strategies, expectations for product design and
development, regulatory applications and approvals, reimbursement
arrangements, costs of sales and market penetration.

By their nature, forward looking statements involve risk and uncertainty
because they relate to future events and circumstances. Forward looking
statements are not guarantees of future performance and the actual
results of the Company’s operations, and the development of its main
product, the markets and the industry in which the Company operates, may
differ materially from those described in, or suggested by, the forward
looking statements contained in this announcement. In addition, even if
the Company’s results of operations, financial position and growth, and
the development of its main product and the markets and the industry in
which the Company operates, are consistent with the forward looking
statements contained in this announcement, those results or developments
may not be indicative of results or developments in subsequent periods.
A number of factors could cause results and developments of the Company
to differ materially from those expressed or implied by the forward
looking statements including, without limitation, the Company’s ability
to obtain CE Marking for ReActiv8®, the initiation and success of the
ReActiv8-B Clinical Trial, general economic and business conditions, the
global medical device market conditions, industry trends, competition,
changes in law or regulation, changes in taxation regimes, the
availability and cost of capital, the time required to commence and
complete clinical trials, the time and process required to obtain
regulatory approvals, currency fluctuations, changes in its business
strategy, political and economic uncertainty. The forward-looking
statements herein speak only at the date of this announcement.